1. Field of the Invention
The present invention generally relates to a system and method for measuring properties of traveling webs of fibrous sheet material during manufacture and, more particularly, to a system and method for providing measurements such as dry basis weight, basis weight, and moisture content of fibrous sheet materials.
2. State of the Art
In the manufacture of sheet materials, important parameters for process and quality control include basis weight and moisture content. Basis weight is normally defined as the weight per unit area of sheet material and is usually stated in units of grams per square meter. Dry basis weight, a related measure that is particulary useful during the manufacture of fibrous sheet materials such as paper and cardboard, refers to weight per unit area of sheet material excluding moisture. In the art of paper-making, dry basis weight is equivalent to the weight of dry material, primarily fibers, comprising a given area of a paper sheet. Moisture content refers to the amount of moisture per unit area of a sheet. These parameters are related by the fact that, for a given area of a sheet, basis weight minus moisture content equals dry basis weight.
It is well known that dry basis weight can be determined by laboratory tests but that such tests have several inherent drawbacks. One shortcoming of laboratory tests is that substantial time is required for sample acquisition and analysis; during that time, substantial quantities of sheet material may be produced and production conditions may change sufficiently that the laboratory test results, when available, may no longer be representative of the conditions of the manufacturing process or product. Another drawback of laboratory tests is that samples obtained for testing may not completely represent sheet material that has been produced; this is because samples are often obtained only at the end of a sheet roll and, therefore, may not be representative of paper quality at intermediate locations along the roll.
To detect the composition of sheet materials, it is known to direct a beam of light or other radiation of known wavelength against a surface of a given area of sheet material and to measure the radiation absorbed by the sheet. Generally speaking, the amount of absorbed radiation at a particular wave length is a function of the composition of the sheet material; for example, the cellulose content of paper sheet material is indicated by the absorption of infrared light having a wavelength of about 1.5 microns.
Various devices have been proposed for making parametric measurements of properties of traveling webs of sheet material on-line. The term "on-line" in this art refers to measurements that are made on a sheet-making machine while the machine is operating. The phrase "parametric measurements" as used herein refers to measurements of physical properties whose values determine characteristics and qualities of the sheet material. Thus, parametric measurements of paper and similar fibrous sheet materials may include basis weight, dry basis weight, moisture content, thickness, and transmissivity.
In paper-making processes, on-line parametric measurements are difficult to make accurately. The difficulties arise, in part, because modern paper-making machines are large and operate at high speeds. For example, some conventional paper-making machines produce sheets which are 100 to 400 inches wide at the rate of about 20 to 100 feet per second. Furthermore, on-line measurements in paper-making processes are often problematical because of severe environmental conditions; for example, the environment around a paper-making machine may include a high concentration of wet pulp and a humid atmosphere that contains water droplets and air-borne particles of sulfuric acid or alkalies. Regardless of such conditions, however, practical on-line measuring devices on sheet-making machines must operate continuously for extended periods and their repair and maintenance must be accomplished without causing substantial down-time to the sheet-making machines.
To provide on-line parametric measurements of paper and other fibrous sheet materials, workers in the art have proposed various sensors that periodically traverse or "scan" traveling webs of sheet material in the cross-direction. (In the sheet-making art, the direction of sheet travel is known as the "machine direction" and the direction across a sheet that is perpendicular to the machine direction is known as the "cross direction"). For example, U.S. Pat. Nos. 3,641,349; 3,681,595; 3,757,122; and 3,886,036 assigned to Measurex Corporation discuss basis weight gauges of the scanning type. Also, U.S. Pat. No. 4,289,964, assigned to Intec Corporation, suggests that beta ray gauges can scan slowly across a traveling web in the cross direction to determine basis weight. Further, the patent to Intec suggests that beta ray gauges can be augmented with laser, infrared, or ultraviolet light sensors to indicate transmissive qualities across the web.
Despite numerous advantages of scanning gauges in sheet-making operations, such gauges have limitations. For example, the moving parts in scanning gages can cause repair and maintenance problems. Also, scanning gauges that require relatively long periods to make successive measurements are not optimal for control purposes that require a relatively large number of measurements over relatively short periods. For instance, conventional scanning gages for detecting the basis weight and moisture content of fibrous sheet materials normally require about thirty seconds to obtain a series of measurements over a traveling sheet in the cross direction. (A series of such measurements in the cross-direction are said to provide a "profile" of the sheet.)
In view of the limitations of scanning devices, it has been proposed to mount a plurality of fixed sensors to detect parameters at locations distributed along the cross-direction of a traveling web of fibrous sheet material. In particular, U.S. Pat. No. 3,806,730 suggests a measuring device that includes a set of tubes mounted to distribute light onto the surface of a moving web in the cross direction. According to the patent, the tubes are rectangular aluminum pipes that have bright interior surfaces to carry light. A set of similar tubes is positioned to receive and detect light transmitted through the web. The device described in the patent is said to be useful for measuring parameters such as basis weight and moisture content of paper sheet materials.
As further background to the present invention, it is useful to generally describe a typical paper-making process. Generally speaking, a paper-making process begins when a slurry of fibers and water, called raw stock, is spread onto a supporting wire mesh from a reservoir called a head box. The wire mesh supports the fibers while allowing substantial drainage. To uniformly spread raw stock onto a wire mesh, it is customary to employ gate-like devices mounted next to one another in the cross direction at the so-called slice lip openings of the head box. Then after raw stock has been discharged through the slice lip openings to form a wet web on a wire mesh, the web is passed through a press section that squeezes water from the web. Then, the web is passed through a dryer section to evaporate water from the web. After the dryer section, the web passes through calendar rollers and, usually, through a scanner and onto a reel. The portion of the paper-making process prior to the dryer section is often referred to as the wet end of the process. It can be appreciated that measurements at the wet end are highly desirable because such measurements may provide early control in the paper production process and, thus, may minimize wastage by indicating needed process changes before substantial quantities of substandard paper are produced. On the other hand, wet end measurements are difficult to make because of the high water content, of paper webs at this stage and severe environmental conditions.